At a Glance

Why Get Tested?

To detect and measure excess fat in the stool; to help diagnose conditions causing malabsorption

When to Get Tested?

When a person has symptoms of malabsorption such as persistent diarrhea and fatty stools

Sample Required?

A random stool collection; sometimes a 72-hour stool collection

Test Preparation Needed?

For a 72-hour stool collection, follow your doctor's instructions. Consume 50-150 grams of fat a day in your diet for 2-3 days prior to and during the stool collection period.

The Test Sample

What is being tested?

This test measures the amount of fat in a stool sample. Excess fecal fat (termed steatorrhea) is an indicator of impaired digestion and/or malabsorption.

The body digests foods in three stages: first proteins, fats, and carbohydrates are broken down, in the stomach by acid and enzymes and in the small intestines by enzymes produced by the pancreas and bile from the liver, into their component parts. They are then absorbed, primarily in the small intestines, and finally the nutrients are transported throughout the body and used or stored.

If there is not enough bile or pancreatic enzymes available, then fat and other foods cannot be properly digested. If a condition prevents the intestines from absorbing nutrients, then they are "lost" by excretion in the stool. In both cases – improper digestion or absorption – the affected person can experience symptoms associated with malabsorption and, in severe cases, symptoms of malnutrition and vitamin deficiency. If the condition prevents the digestion and/or absorption of fats from the diet, then excess fat is present in the stool and the person may experience prolonged diarrhea with stomach pain, cramps, bloating, gas and weight loss.

Fat in stool may be detected with the qualitative fecal fat test, which generally determines the presence or absence of excess fat. This is the simplest test for fecal fat and is performed by placing a suspension of stool onto a glass slide, adding a fat stain, and observing the number and size of fat globules that are present.

Quantitative measurements of fecal fat, though somewhat more precise, are less commonly performed. They require a timed stool collection and a dietary sheet to help calculate the total fat intake during the collection period, usually a 72-hour stool collection. Results are reported as the amount of fat excreted per 24 hours. A variation of the test is called the acid steatocrit, which provides a rapid but less exact measure of the amount of fat in the stool.

How is the sample collected for testing?

For a 72-hour stool collection, save all stool during the collection time period in the container(s) provided. It may be collected in a variety of ways, such as on plastic wrap, but should not be contaminated by urine, toilet water, or toilet paper. For a random sample, collect a single specimen in the same fashion.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

For a 72-hour stool collection, consume 50-150 grams of fat a day for 2-3 days prior to and during the stool collection period. The fat should be long-chain triglycerides (such as corn or olive oil, not butter). Follow the doctor's instructions and dietary recommendations.

The Test

How is it used?

Fecal fat testing is typically ordered along with or as a follow up to other stool tests and blood tests to investigate the cause of chronic diarrhea and loose, fatty, foul-smelling stools (steatorrhea).

Fecal elastase – a protein-cleaving enzyme produced and secreted by the pancreas; it is resistant to degradation by other enzymes and so is excreted and can be measured in the stool. The amount of this enzyme is reduced in pancreatic insufficiency.

Fecal fat, qualitative – usually a suspension of stool placed on a glass slide that is treated with a special stain and examined under the microscope to detect the presence of fat droplets

Blood tests may be ordered to further investigate malabsorption symptoms and may include:

If the qualitative fecal fat is negative, then a 72-hour quantitative fecal fat test may be ordered. This is a better evaluation of fat digestion and absorption. There are two reasons for this:

For the quantitative test, the person being tested is required to ingest a moderately high amount of fat per day prior to and during sample collection so their absorption ability is being "challenged."

Fat is not excreted at a constant rate in the stool so the combination and mixing of the stool from a 72-hour collection gives a more accurate picture of average absorption and excretion than a single sample.

What does the test result mean?

A positive qualitative fecal fat test or an increased amount of fat in a 72-hour quantitative fecal fat test indicates that fat is likely not being absorbed normally and that the person may have impaired digestion or malabsorption.

Malabsorption is seen with a wide variety of diseases and conditions. Some causes of malabsorption include:

Narrowing or blockage of the common bile duct, the main tube that carries bile from the liver and gallbladder to the intestines

Other laboratory tests used in conjunction with the fecal fat test are usually required to determine the underlying cause of fat malabsorption.

A negative qualitative fecal fat test does not necessarily rule out malabsorption so it is often followed up with a quantitative test.

In a 72-hour fecal fat test, a low level of fecal fat excretion per day generally indicates that the person tested is digesting and absorbing fats normally and suggests that the symptoms being experienced are likely due to another cause.

Is there anything else I should know?

Children cannot ingest as much daily fat as adults. Their test preparation will be adjusted and their 72-hour fecal fat test results will typically be reported as a percentage. This result is a "coefficient" that compares the amount of fat eaten to the amount excreted in order to evaluate the quantity of fat absorbed.

Although 72 hours is the most typical sample collected for a quantitative fecal fat, a doctor may sometimes ask for a 24- or 48-hour stool sample instead.

1. If my doctor tells me to eat 100 grams of fat a day, can I vary it between 50 and 150 grams?

You should try to follow your doctor's recommendations as closely as possible. Eating a similar amount of fat each day will help ensure that the interpretation of your results is accurate. For more information on the fat content of various foods, visit the page on Food Exchange Lists by the National Heart, Lung and Blood Institute

3. Can the fecal fat test be done in my doctor's office?

The qualitative fecal fat test may be done in the doctor's office if it has the proper stain and equipment. The quantitative test requires specialized equipment. It needs to be performed by a laboratory and may need to be sent to a reference laboratory.

Article Sources

NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

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This article was last reviewed on September 26, 2011. | This article was last modified on January 27, 2012.

The review date indicates when the article was last reviewed from beginning to end to ensure that it reflects the most current science. A review may not require any modifications to the article, so the two dates may not always agree.

The modified date indicates that one or more changes were made to the article. Such changes may or may not result from a full review of the article, so the two dates may not always agree.